Project Summary Relapse following treatment for substance use disorder (SUD) remains the norm, not the exception. Mindfulness treatment is a particularly efficacious outpatient aftercare approach for reducing relapse among individuals stepping down from more intensive SUD treatment (e.g., residential or intensive outpatient). Unfortunately, aftercare mindfulness treatment is inaccessible to most patients, due to limited clinician training opportunities and a lack of effective implementation strategies. For example, to date there are no existing technology-based (e.g., web-based) aftercare mindfulness treatments for preventing relapse to substance use. Research is also lacking on mechanisms of change in mindfulness treatment for SUD (i.e., how and why this treatment works). Ecological momentary assessment (EMA), or intensive longitudinal measurement via smartphone, captures dynamic, high-resolution data in near real-time. Mindfulness, negative/positive affect, stress, and craving - which are momentary states that change over the course of a day - are central to putative mechanisms of mindfulness treatment for SUD. Mindfulness treatment for SUD is theorized to weaken or ?decouple? the momentary associations of negative affect with craving, stress with craving, and (low) positive affect with craving. However, no studies to date have used EMA to evaluate these putative momentary decoupling mechanisms following mindfulness treatment for SUD. The proposed K23 career development award provides training to support Dr. Corey Roos in becoming an independent patient-oriented researcher focused on technology-based addiction treatment and mechanisms of change in addiction recovery. The training goals of this award focus on technology-based treatment development, rigorous clinical trial research, EMA, and mechanisms of mindfulness. Dr. Roos will receive training in these goals through direct mentorship from experts, coursework, conferences, and hands-on research activities. The proposed research project of this K23 application involves developing and evaluating UrgeSurfer, the first web-based mindfulness treatment for preventing relapse substance use among individuals with SUDs. The web program will be iteratively developed through a user-centered design framework involving client feedback and usability testing. We will then conduct a pilot randomized controlled trial to evaluate UrgeSurfer as an adjunctive outpatient aftercare treatment for preventing relapse. Moreover, as part of this trial, will investigate putative decoupling mechanisms of change using EMA. This K23 award will propel Dr. Roos into a successful career as an independent patient-oriented addiction treatment researcher.